Chlorine and caustic are essential, large volume commodities which are basic chemicals required by all industrial societies. They are produced almost entirely electrolytically from aqueous solutions of alkali metal halides or, more particularly, sodium chloride, with a major portion of such production coming from diaphragm-type electrolytic cells. In the diaphragm electrolytic cell process, brine (saturated sodium chloride solution) is fed continuously to the anode compartment to flow through a diaphragm usually made of asbestos particles formed over a cathode structure of a foraminous nature. To minimize back migration of the hydroxide ions, the flow rate is always maintained in excess of the conversion rate so that the resulting catholyte solution has unused or unreacted sodium chloride present. Hydrogen ions are discharged from the solution at the cathode in the form of hydrogen gas. The catholyte solution containing caustic soda (sodium hydroxide), unreacted sodium chloride and other impurities, must then be concentrated and purified to obtain a marketable sodium hydroxide commodity. The unreacted sodium chloride is returned to the electrolytic cells for reuse in further production of sodium hydroxide and chlorine. The evolution of hydrogen gas requires a high voltage thereby reducing the power efficiency possible from such an electrolytic cell thus creating an energy inefficient means of producing sodium hydroxide and chlorine gas.
With the advent of technological advances such as dimensionally stable anodes and various coating compositions therefor which permit ever narrowing gaps between electrodes, the electrolytic cell has become more efficient in that the power efficiency is greatly enhanced since electrolyte resistance in the narrow anode/cathode gap is reduced. Also, the hydraulically impermeable membrane has added a great deal to the use of electrolytic cells in terms of selective migration of various ions across the membrane so as to exclude contaminants from the resultant product thereby eliminating at least some of the costly purification and concentration steps required in the processing of diaphragm cell products.
The largest advancements in electrolytic cell technology have tended to improve the efficiency of the anodic side and the membrane or seperator portion of electrolytic cells. Currently, more attention is being directed to the cathodic side of the electrolytic cell in an effort to improve the power efficiency of the cathodes to be utilized in the process and to create a significant energy savings in the cathode reaction process.
In a conventional chlorine and caustic cell, employing a conventional anode and cathode and a diaphragm seperator therebetween, the electrolytic reaction at the cathode may be represented as EQU 2H.sub.2 O+2e.sup.- yields H.sub.2 +2OH (1)
The discharge potential of this reaction as measured against a standard hydrogen electrode is -0.83 volts. The desired reaction under ideal circumstances to be promoted at the cathode would be EQU 2H.sub.2 O+O.sub.2 +4e.sup.- yields 4OH.sup.- ( 2)
The potential for this reaction is +0.40 volts. The use of this reaction as opposed to the common hydrogen discharge reaction would result in a theoretical voltage savings of 1.23 volts. The electrical energy necessarily consumed to produce the hydrogen gas which is an undesirable reaction product of the cathode in conventional electrolytic cells has not been counterbalanced efficiently in the industry by the utilization of the resultant hydrogen. While some uses have been made of the excess hydrogen gas, those uses have not made up the difference in expenditure of electrical energy necessary to evolve the hydrogen. Thus, if the evolution of hydrogen gas could be substantially reduced or eliminated from the electrolysis process, it would save electrical energy and make production of chlorine and caustic more energy efficient, while avoiding the separation and disposal problems associated with the production of hydrogen.
The oxygen electrode presents one possibility for the elimination of the production of hydrogen since it consumes oxygen to combine with water and the electrons available at the cathode in accordance with the following equation EQU 2H.sub.2 O+O.sub.2 +4e.sup.- yields 4OH.sup.- ( 3)
It is readily apparent that this reaction is more energy efficient by the very absense of the production of any hydrogen at the cathode and at least theoretically affords the reduction in potential as shown above. Oxygen electrodes are normally porous materials and the reaction is accomplished by feeding an oxygen-rich fluid such as air or pure oxygen to one side of the oxygen electrode where the oxygen has ready access to the electrolytic surface in contact with the electrolyte so as to be consumed in accordance with the above equation. This does, however, require a significantly different structure for the electrolytic cell itself so as to provide for an oxygen compartment on one side of the cathode so that the oxygen-rich fluid may be fed thereto.
Oxygen electrodes have become well-known in the art since many NASA projects to promote space travel during the 1960's also provided funds for the development of a fuel cell utilizing an oxygen cathode and a hydrogen anode to produce electrical current for utilization in a spacecraft by feeding hydrogen and oxygen gas to the electrodes to make water. While this major, government-financed research effort produced many fuel cell components including an oxygen electrode, the circumstances and the environment in which the fuel cell oxygen electrode functions are quite different from that which is experienced in a chlor-alkali cell. Thus, while much of the technology gained during the NASA projects is of value in the chlor-alkali industry with regard to the development of a oxygen electrode, much further development has been necessary to adapt the oxygen electrode to the chlor-alkali cell cathode environment.
Some attention has been given to the use of an oxygen cathode in a chlor-alkali cell so as to increase the efficiency in the manner described to be theoretically feasible, but thus far, the oxygen cathode has failed to receive significant interest so as to produce a commercially effective or economically viable electrode for use in an electrolytic cell to produce chlorine and caustic. While it is recognized that a proper oxygen cathode will be necessary to realize the theoretical efficiencies to be derived therefrom, the chlor-alkali cell will require an electrode significantly different from that of a fuel cell since the electrical potential will be applied to the chlor-alkali cell for the production of chlorine and caustic rather than electrical potential being drawn from the electrodes as in a fuel cell. Therefore, it would be advantageous to develop an oxygen cathode which will approach the theoretical electrical efficiencies possible with an ideal oxygen electrode in the cathode compartment of a chlor-alkali electrolytic cell.
In order to operate efficiently and maintain a reasonable lifetime in a cell environment, the electrolyte should penetrate into the electrode sufficiently to reach the interior surfaces of the electrode and thereby contact the gas in as many places as possible in the presence of the electrode and any catalyst associated therewith. However, the electrode must be sufficiently hydrophobic to prevent the electrolyte from flooding the pores of the electrode and "drowning" the electrode. When drowning occurs, the reaction zone is moved away from the electrolyte side of the electrode deeper into the interior of the electrode. This results in some electrolyte being relatively immobile within the pores of the electrode and somewhat separated from the main body of the electrolyte. Thus, the ions formed by the cell reaction in the interior portions of the electrode cannont readily escape from the reaction zone of the electrode and cell performance drops. This build-up of ions in the reaction zone and the resultant decrease in cell performance is known as "concentration polarization."
There have been many attempts to provide a gas electrode which permits good gas-electrolyte-electrode contact without drowning or polarizing the electrode. It has been proposed to make pores of the electrode smaller on the electrolyte side of the electrode body than those on the gas side of the body so that the combined effect of the surface tension of the liquid electrolyte in the small pores and the pressure of the gas from the opposite side of the electrode prevents the electrolyte from flooding that portion of the electrode having the larger pores. This requires precise gas pressure control which increases the size and weight of the cell. Furthermore, it is difficult to obtain an electrode having a uniform gradient of pore size ranging from large on the gas side to small on the electrolyte side.
Other methods of improving cell performance have included attempts to wet-proof the electrode such as by dipping the electrode in dilute solutions of wax in a low-boiling solvent. By this method, the electrode is rendered somewhat hydrophobic but the wax can block the electrode pores and/or insulate the electrode surface from the desired reaction.
While electrodes for use in fuel cells have not found commercial utility in chlor-alkali electrolysis, their development is pertinent to the search to obtain a viable electrode in a chlor-alkali cell environment. Thus, Reutschi, U.S. Pat. No. 3,062,909, discloses an oxygen electrode comprising a nickel-silver-paladium powder mixture which is sintered to form a porous electrode. Additionally, a metal screen or expanded metal may be incorporated within the sintered mass of metal powder to lend strength to the electrode while not inhibiting the passage of gas through the electrode.
Kometani, et al, U.S. Pat. No. 3,329,530, describes a sintered fuel cell electrode comprising 50 to 95% by volume of a conductive material such as carbon or nickel and from 5 to 50% by volume of a hydrophobic binder component such as polytetrafluoroethylene (PTFE). The electrode is formed by pressing a powder mixture of the components in a mold and then sintering the resultant article at a temperature substantially higher than the melting point of the binder component. No pressure is utilized during the sintering step, however.
LeDuc, U.S. Pat. No. 3,400,019, describes an electrode having a non-metallic substrate such as a polymer material, ceramic material or graphite, having thereon a film of electroconductive metal which is preferably applied by electroplating.
Carson, et al, U.S. Pat. No. 3,415,689, describes an oxygen electrode wherein a spinel catalyst and PTFE mixture is applied to a porous graphite electrode substrate. Preferred spinel catalysts are cobalt aluminate, magnesium aluminate, silver ferroso-ferric oxide and nickle ferrate. The spinel mixture is applied by a painting process on the graphite substrate.
Darland et al, U.S. Pat. No. 3,423,247, describes an electrode having a microporous high surface area catalyzed layer on the electrolyte side of the electrode and a low surface area non-catalyzed, highly hydrophobic area on the gas side of the electrode. With this structure, gas is able to penetrate the macroporous gas side of the electrode while electrolyte is not able to penetrate this area from the opposite side. This condition creates a reaction zone in the central portion of the electrode and avoids flooding and the consequent failure of the electrode.
In Giner, U.S. Pat. No. 3,438,815, an oxygen electrode is produced by applying a coating of nobel metal black and PTFE in an aqueous solution which is dried and sintered onto a porous metal substrate, the metal being selected from nickel, copper, valve metals, or noble metals. The metal substrate layer may be produced by sintering a mixture of metal powder and ceramic carrier to produce the porous structure.
Deibert, U.S. Pat. No. 3,457,113, describes a laminar electrode having a hydrophobic layer of carbon and polymer laminated with a hydrophilic layer of metal catalyst and electroconductive material. Optionally, a metal screen may be pressed into the laminate in order to strengthen the resultant electrode. The laminate layers are produced by fusion of the binder component with heat and/or pressure.
U.S. Pat. No. 3,600,230, Stachurski, describes a gas-depolarized electrode comprising a metallic grid or screen upon which a layer of hydrophobic resinous material and fiberous conductive material has been formed to create a surface upon which a second layer of catalytically active material such as platinum or silver is formed using a hydrophobic resinous material as a binder.
In Binder, U.S. Pat. No. 3,854,994, a gas electrode is produced by filtering a slurry of polytetraflurorethylene powder to obtain a filter cake followed by the step of drawing a solution of carbon powder, graphite fibers and polytetrafluoroethylene through such filter cake to form a second layer on the filter cake first layer. The electrode is then dried and heated to about 330.degree. C. in a non-oxidizing atmosphere. The filter cake is formed on a metal screen of electroconductive, corrosion resistant material.
Gritzner, U.S. Pat. No. 3,923,628, describes a chlor-alkali cell having an oxygen cathode comprising a silver plated copper screen substrate coated with a mixture of platinum black, silver balck or carbon black with PTFE or other fluorinated hydrophobic polymer. Platinum screening may be substituted for copper screening as substrate material. The high cost of these materials has prevented commercial application of this chlor-alkali cell even though a 200 to 800 millivolt advantage (depending on current density) is indicated by the patent.
None of the above electrodes has found commercial utility in the production of chlorine and caustic in an electrolytic cell. The principal limiting factors have been cost of the electrode material, particularly those employing large amounts of precious metals, and electrode life span in the highly corrosive environment of the cathode compartment of a chlor-alkali electrolytic cell.
It is therefore a principal object of this invention to provide a gas-depolarized electrode for use as a cathode in a chlorine and caustic cell which has sufficient porosity and hydrophobicity for efficient oxygen reduction while having a structural integrity which permits extended life in the corrosive environment of a chlor-alkali cell.
It is another object of this invention to reduce the cost of a gas-depolarized cathode for use in a chlorine and caustic cell through the utilization of common electrocatalytic materials employing only small amounts of precious metals.
These and other objects of the invention are accomplished by a novel electrode and process of making same to be described hereinafter.